Recently, there have been reported increasing levels of active researches on holographic digital data storage systems triggered by the development of semiconductor lasers, charge coupled devices (CCDs), liquid crystal displays (LCDs) and the like. Since the holographic digital data storage system normally features a large storage capacity and high data transfer rate, it has already been applied to, e.g., fingerprint recognition systems for storing and reproducing fingerprints, and the scope of its applications keeps expanding.
The holographic digital data storage system allows a signal beam transmitted from an object to interfere with a reference beam, and writes interference patterns generated from such interference phenomena on a storage medium such as a crystal or a photopolymer which reacts differently depending on the amplitude and phase of an interference pattern. In the holographic digital data storage system, the phase of the signal beam as well as the amplitude thereof may be recorded by changing an incident angle of the reference beam, so that a three dimensional display of an object can be realized. Further, hundreds to thousands of holographic digital data comprised of binary data on a page-by-page basis can be stored in a single space of the storage medium.
FIG. 11 depicts an overall block diagram of a holographic digital data storage system, wherein the holographic digital data storage system comprises a light source 20, a beam expander 21, a beam splitter 22, two reflection mirrors 23 and 24, a spatial light modulator (SLM) 25, a medium 26 and a CCD 27.
The light source 20 generates an optical signal, e.g., a laser beam, whose wavelength falls within a specific wavelength band required for the holographic digital data. The beam expander 21 expands the size of the laser beam.
The beam splitter 22 separates the expanded laser beam into a reference beam and a signal beam and transfers the reference beam and the signal beam through two different transmission channels, wherein the reference beam and the signal beam correspond to a transmitted beam and a reflected beam, respectively.
The reference beam is reflected at the reflection mirror 24 so that the reflected reference beam is transferred to the medium 26. The signal beam, on the other hand, is reflected at the reflection mirror 23 so that the reflected signal beam is transferred to the SLM 25. The SLM 25 modulates the reflected signal beam into binary pixel data on a page basis. The modulated signal beam is transferred to the medium 26. In case the reflected signal beam is, for example, image data provided on a frame basis, the reflected signal beam is preferably modulated on a frame basis and the reflection mirror 24 functions to change the reflection angle of the reflected reference beam by a small amount.
The medium 26 stores the interference pattern acquired from an interference phenomenon between the reflected reference beam and the modulated signal beam, wherein the interference pattern depends on the reflected signal beam, i.e., the data inputted to the SLM 25. In other words, the modulated signal beam irradiated to the medium 26 is modulated on a page basis and the reflected reference beam is reflected in an angle corresponding to the modulated signal beam. The modulated signal beam interferes with the reflected reference beam within the medium 26. The amplitude and phase of the interference pattern results in a photo-induction within the medium 26 so that the interference pattern may be written on the medium 26.
When only the reference beam is irradiated onto the medium 26 in order to reconstruct the data written thereon, the reference beam is diffracted by the interference pattern within the medium 26 so that a check pattern with original brightness on a pixel basis may be restored. When the check pattern is irradiated on the CCD 27 in turn, the original data may be restored. The reference beam used for reproducing the data written on the medium 26 should be irradiated at the same incident angle as that of the reference beam when recording the data on the medium 26.
FIG. 12 presents a block diagram of a conventional CD or DVD player, wherein the CD/DVD player comprises a high frequency overlap module 10, two mirrors 11 and 18, a polarizing prism 12, a cylindrical lens 13, a photodiode (PD) 14, a λ/4 plate 15, a disc medium 16, an object lens 17 and a collimating lens 19. A detailed description for the structure and the operational principle of such CD/DVD player will be omitted here since it is well known to a person having ordinary skill in the relevant art.
As for the conventional CD/DVD player of FIG. 12 and the conventional holographic digital data storage system of FIG. 11, however, there has been found a drawback in that they cannot be compatible with each other since the positions of their detectors, e.g. optical diodes, are different from each other. To be specific, since the CD/DVD player has its detector along a direction of reflection while the holographic digital data storage system has its detector along a transmission direction, a single detector cannot be used for both systems. Further, the size difference of beams used in the two systems is so great that two different optical instruments are required.